Conventionally, in order to protect a device from high voltage such as high voltage lightning surge, switching surge, or static electricity that enters through a signal line or a power supply line, there have been widely used a varistor formed of a high resistive element having nonlinear voltage characteristics and an arrester having a discharge gap accommodated in a tight container. In the case of a low voltage signal line, a suppression voltage of the varistor or the arrester cannot be set to be as low as about several volts, and therefore a Zener diode capable of setting the suppression voltage to be lower is used between signal lines as a surge protector (for example, Patent Literatures 1 and 2 below).